I've seen it theorized that SteamOS was less a flop and more a shot across Microsoft's bow that had the intentional effect. It briefly seemed like a war was brewing over the Windows Store or the Steam Store being the main outlet for PC game purchases, particularly games developed by MS studios. Seems that all but disappeared when the Steam Machine came out.
SteamOS has nothing to do with the Windows Store failing. That was tied to WinRT and Windows Phone. SteamOS and the few Steam Machine designs announced were underwhelming junk that had no market.
But you could be absolutely right that it was intended as a signal to Microsoft. Maybe Valve just got lucky and didn't have to follow through seriously.
It's more like an insurance policy than a serious attempt at building a platform. However, should it come to it, a lot of the problems with desktop Linux really come from a lack of money and direction - Valve have the liquidity, and games are enough of a selling point to justify only supporting one or two distros.
